PF: Shortcuts For Creating Rulesets

Table of Contents

Introduction

PF offers many ways in which a ruleset can be simplified. Some good
examples are by using
macros and
lists.
In addition, the ruleset language, or grammar, also offers some
shortcuts for making a ruleset simpler. As a general rule of thumb, the
simpler a ruleset is, the easier it is to understand and to maintain.

Using Macros

Macros are useful because they provide an alternative to hard-coding
addresses, port numbers, interfaces names, etc., into a ruleset. Did a
server's IP address change? No problem, just update the macro; no need
to mess around with the filter rules that you've spent time and energy
perfecting for your needs.

A common convention in PF rulesets is to define a macro for each
network interface. If a network card ever needs to be replaced with one
that uses a different driver, for example swapping out a 3Com for an
Intel, the macro can be updated and the filter rules will function as
before. Another benefit is when installing the same ruleset on multiple
machines. Certain machines may have different network cards in them,
and using macros to define the network interfaces allows the rulesets to
be installed with minimal editing. Using macros to define information in
a ruleset that is subject to change, such as port numbers, IP addresses,
and interface names, is recommended practice.

If the internal network ever expanded or was renumbered into a different
IP block, the macro can be updated:

IntNet = "{ 192.168.0.0/24, 192.168.1.0/24 }"

Once the ruleset is reloaded, everything will work as before.

Using Lists

Let's look at a good set of rules to have in your ruleset to handle
RFC 1918
addresses that just shouldn't be floating around the Internet,
and when they are, are usually trying to cause trouble:

block in quick on tl0 inet from 127.0.0.0/8 to any
block in quick on tl0 inet from 192.168.0.0/16 to any
block in quick on tl0 inet from 172.16.0.0/12 to any
block in quick on tl0 inet from 10.0.0.0/8 to any
block out quick on tl0 inet from any to 127.0.0.0/8
block out quick on tl0 inet from any to 192.168.0.0/16
block out quick on tl0 inet from any to 172.16.0.0/12
block out quick on tl0 inet from any to 10.0.0.0/8

Now look at the following simplification:

block in quick on tl0 inet from { 127.0.0.0/8, 192.168.0.0/16, \
172.16.0.0/12, 10.0.0.0/8 } to any
block out quick on tl0 inet from any to { 127.0.0.0/8, \
192.168.0.0/16, 172.16.0.0/12, 10.0.0.0/8 }

The ruleset has been reduced from eight lines down to two.
Things get even better when macros are used in conjunction with
a list:

NoRouteIPs = "{ 127.0.0.0/8, 192.168.0.0/16, 172.16.0.0/12, \
10.0.0.0/8 }"
ExtIF = "tl0"
block in quick on $ExtIF from $NoRouteIPs to any
block out quick on $ExtIF from any to $NoRouteIPs

Note that macros and lists simplify the pf.conf file, but
the lines are actually expanded by
pfctl(8) into multiple rules. So, the above example actually
expands to the following rules:

block in quick on tl0 inet from 127.0.0.0/8 to any
block in quick on tl0 inet from 192.168.0.0/16 to any
block in quick on tl0 inet from 172.16.0.0/12 to any
block in quick on tl0 inet from 10.0.0.0/8 to any
block out quick on tl0 inet from any to 10.0.0.0/8
block out quick on tl0 inet from any to 172.16.0.0/12
block out quick on tl0 inet from any to 192.168.0.0/16
block out quick on tl0 inet from any to 127.0.0.0/8

As you can see, the PF expansion is purely a convenience for the writer
and maintainer of the pf.conf file, not an actual
simplification of the rules processed by
pf(4).

Macros can be used to define more than just addresses and ports; they
can be used anywhere in a PF rules file:

pass in quick on ep0 inet proto tcp from 198.51.100.80 to any \
port = 80
pass in quick on ep0 inet proto tcp from 198.51.100.80 to any \
port = 6667
pass in quick on ep0 inet proto tcp from 203.0.113.79 to any \
port = 80
pass in quick on ep0 inet proto tcp from 203.0.113.79 to any \
port = 6667
pass in quick on ep0 inet proto tcp from 203.0.113.178 to any \
port = 80
pass in quick on ep0 inet proto tcp from 203.0.113.178 to any \
port = 6667

PF Grammar

Packet Filter's grammar is quite flexible which, in turn, allows
for great flexibility in a ruleset. PF is able to infer certain
keywords which means that they don't have to be explicitly stated in a
rule, and keyword ordering is relaxed such that it isn't necessary to
memorize strict syntax.

Elimination of Keywords

To define a "default deny" policy, two rules are used:

block in all
block out all

This can now be reduced to:

block

When no direction is specified, PF will assume the rule applies to
packets moving in both directions.

Similarly, the "from any to any" and "all" clauses can
be left out of a rule, for example:

block in on rl0 all
pass in quick log on rl0 proto tcp from any to any port 22 keep state

can be simplified as:

block in on rl0
pass in quick log on rl0 proto tcp to port 22 keep state

The first rule blocks all incoming packets from anywhere to anywhere on
rl0, and the second rule passes in TCP traffic on rl0 to port 22.

Return Simplification

A ruleset used to block packets and reply with a TCP RST or ICMP
Unreachable response could look like this:

block in all
block return-rst in proto tcp all
block return-icmp in proto udp all
block out all
block return-rst out proto tcp all
block return-icmp out proto udp all

This can be simplified as:

block return

When PF sees the return keyword, it's smart enough to send the
proper response, or no response at all, depending on the protocol of the
packet being blocked.

Keyword Ordering

The order in which keywords are specified is flexible in most cases. For
example, a rule written as: